1. Field of the Invention
The present invention relates to an image fixing apparatus which fixes an image on a sheet, and an image forming apparatus having the image fixing apparatus.
2. Related Background Art
Generally, an image forming apparatus fixes an unfixed image, drawn on a sheet by toner, on the sheet by heating and pressurizing in a fixing unit. The fixing unit is heated by an internal heater, and is controlled to maintain temperature necessary for fixing, with compensating heat amount taken by the sheet to pass.
Now, types of sheet material transferred by an image forming apparatus increase every year, and it is hard to reconcile the fixability, image quality of a fixed image, and productivity, which are stabilized to all the material, with each other in the structure of one fixing unit performing image fixing. What is adopted so as to correspond to this is a method that a plurality of fixing units are located in series in a conveying path to avoid the problems resulting from one fixing unit structure including the lack of heat amount (refer to Japanese Patent Application Laid-Open No. H07-271226).
In addition, the structure which is also disclosed is one that a first fixing portion which fixes toner to a print medium in a glossy state, and a second fixing portion which fixes toner to a print medium in a lusterless state are provided, and a recording medium is selectively conveyed to either of the two fixing portions (refer to Japanese Patent Application Laid-Open Nos. 2002-372882 and H06-348159).
Furthermore, there is material, whose fixability can be satisfied by one fixing unit and which causes problems of curling and coiling around a fixing roller when surplus heat amount is applied, such as paper with small basic weight, which is called plain paper, and a second side of thick paper (a moisture content drops and sheet temperature also rises). What are disclosed in U.S. Pat. No. 6,512,914 and Japanese Patent Application Laid-Open No. 2001-005319 so as to correspond to such material are the structure that an additional conveying path (bypass conveying path) which branches from a conveying path (main conveying path), where two fixing units are provided, to bypass a fixing unit in a downstream side between the two fixing units is provided.
Nevertheless, there is a possibility that, in such structure, intervals of sheets which are regularly conveyed in equal sheet intervals till a point of entering a bifurcation between the main conveying path and bypass conveying path become uneven in the downstream of the confluence between the main conveying path and bypass conveying path. Sheet conveyance in such uneven intervals interferes with the motion control of sheets, and causes a malfunction, for example, when a sheet reversing mechanism, a double-side path, a post treating apparatus, and the like are provided in the downstream of the confluence. In addition, there is also a possibility that a preceding sheet and a subsequent sheet may collide in the confluence to generate a jam, according to the relationship between the relative difference between path lengths and paper interval distance.
In addition, when wide initial sheet intervals are taken before inrush into the fixing unit so that a minimum sheet interval necessary for control is secured even if a sheet passes any of the main conveying path and bypass conveying path, there arises a problem that the productivity of an apparatus drops in the case of a job which uses both the main conveying path and bypass conveying path.